Method of cutting rare earth alloy

ABSTRACT

A method of cutting a rare-earth alloy with a wire saw, obtained by fixing abrasive grains on a core wire with a resin layers, includes the step of moving the wire saw while a portion of the rare-earth alloy being machined with the wire saw is immersed in a coolant, which is mainly composed of water and has a surface tension of about 25 mN/m to about 60 mN/m at about 25° C., thereby cutting the rare-earth alloy. In the wire saw, an average distance between two of the abrasive grains, which are adjacent to each other in a length direction, is about 150% to less than about 400% of the average grain size of the abrasive grains, an average height of portions of the abrasive grains, protruding from the surface of the resin layer, is about 70% or less of the average grain size of the abrasive grains, and a thickness deviation percentage of the resin layer with respect to the core wire is about 40%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of cutting a rare-earth alloyand more particularly, the present invention relates to a method ofcutting a rare-earth alloy with a wire saw, which is obtained by fixingabrasive grains on a core wire.

2. Description of the Related Art

A rare-earth alloy is used as a material to make a strong magnet. Arare-earth magnet, obtained by magnetizing a rare-earth alloy, can beused effectively as a magnet for a voice coil motor, which is used toposition a magnetic head in a magnetic recorder, for example.

In the prior art, a rare-earth alloy material (e.g., in the form of aningot or a sintered body) is often cut by a technique of slicing thematerial with a rotating slicing blade, for example. In this slicingblade cutting process, however, the cutting edge is relatively thick,thus requiring a lot of machining allowance. As a result, the yield ofthe rare-earth alloy material is so low that it increases the cost ofresultant rare-earth alloy products (e.g., rare-earth magnets).

A wire saw cutting process is known as a cutting method that requiressmaller machining allowance than the slicing blade cutting process does.For example, Japanese Laid-Open Publication No. 11-198020 discloses thata hard and brittle material such as silicon, glass, neodymium or ferritemay be cut with a wire saw, which is obtained by fixing superabrasivegrains on the outer surface of a high-hardness core wire with a bondinglayer (which will be referred to herein as a “fixed abrasive wire saw”).

If a number of plates with a predetermined thickness can be obtained atthe same time by cutting a rare-earth alloy material with such a fixedabrasive wire saw with small machining allowance, then the manufacturingcost of rare-earth magnets can be reduced significantly. However, nobodyhas ever reported that a rare-earth alloy could be cut successfully withsuch a fixed abrasive wire saw at a mass-producible level.

The present inventors carried out extensive research on this phenomenonand discovered the major cause of this problem in a significantdifference in mechanical property between a rare-earth alloy produced bya sintering process (which will be referred to herein as a “rare-earthsintered alloy”) and silicon, for example. More specifically, arare-earth sintered alloy includes an overall hard and brittle mainphase (i.e., R₂Fe₁₄B crystal grains) and a grain boundary phase thatcauses ductile fracture. Accordingly, unlike a hard and brittle materialsuch as silicon, the rare-earth sintered alloy is not so easy to cut.That is to say, compared with cutting silicon or any other hard andbrittle material, higher cutting resistance is produced and a hugequantity of heat is generated, too. Also, the specific gravity of arare-earth alloy is approximately 7.5, which is much higher than that ofsilicon or any other hard and brittle material. For that reason, cuttingdebris (or sludge), produced by the machining process, cannot be easilyflushed away from the machined portion.

Thus, to cut a rare-earth alloy with high machining accuracy andefficiency, it is necessary to not only decrease the cutting resistancesufficiently but also efficiently dissipate the heat to be generatedduring the cutting process (i.e., efficiently cool the machinedportion). Furthermore, it is also necessary to efficiently flush awaythe cutting debris produced by the cutting process.

For that purpose, by supplying the rare-earth alloy machined portionwith plenty of highly lubricating coolant (which will also be referredto herein as a “cutting fluid”), the cutting resistance can be decreasedand the heat generated during the cutting process can be dissipatedefficiently. The present inventors discovered and confirmed viaexperiments that if a wire saw is wet with a sufficient amount of an oilcoolant, then the traveling wire saw can supply a narrow machinedportion with plenty of that coolant.

When such an oil coolant is used, however, it costs a lot to process itswaste so as not to create any environmental damage and it is difficultto recycle the waste or cutting debris because the cutting debris ishard to separate from the waste. In view of these considerations, water(or an aqueous coolant) is preferably used as the coolant. However, ifwater is used as a coolant, then it is impossible to keep a sufficientamount of water deposited on the traveling wire saw because water haslow viscosity (with a kinematic viscosity of 1.0 mm²/s). As a result,even if the wire saw was wet with water, a sufficient amount of watercould not be supplied to the machined portion.

Japanese Laid-Open Publication No. 11-198020 discloses that by movingthe wire saw through a coolant overflowing from a coolant vessel, thecoolant can be kept deposited on the wire saw just as intended even in asituation where a fixed abrasive wire saw needs to travel at a highvelocity (e.g., at 2,000 m/min). However, the present inventorsdiscovered via experiments that even when a rare-earth alloy was cutwith a wire saw traveling through such overflowing water (as disclosedin Japanese Laid-Open Publication No. 11-198020, for example), theabrasive grains still dropped off, the resin layer peeled off, or thewire saw snapped in a worst case scenario. These inconveniences alsohappened even when the wire saw traveled at a velocity of about 800m/min.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention extend the life of a wire saw when a rare-earthalloy is cut with a wire saw machine using a coolant mainly composed ofwater.

A rare-earth alloy cutting method according to preferred embodiments ofthe present invention is a method of cutting a rare-earth alloy with awire saw obtained by fixing abrasive grains on a core wire with a resinlayer. The method includes the step of moving the wire saw while aportion of the rare-earth alloy being machined with the wire saw isimmersed in a coolant, which is mainly composed of water and has asurface tension of about 25 mN/m to about 60 mN/m at about 25° C.,thereby cutting the rare-earth alloy. In the wire saw, an averagedistance between two of the abrasive grains, which are adjacent to eachother in a length direction, is about 150% to less than about 400% ofthe average grain size of the abrasive grains, an average height ofportions of the abrasive grains, protruding from the surface of theresin layer, is about 70% or less of the average grain size of theabrasive grains, and a thickness deviation percentage of the resin layerwith respect to the core wire is about 40%, thereby achieving theadvantages described above.

A coolant mainly composed of water for use in another rare-earth alloycutting method according to a preferred embodiment of the presentinvention may also be specified by its kinetic friction coefficient, notthe surface tension thereof. In that case, a coolant, having a kineticfriction coefficient of about 0.1 to about 0.3 at about 25° C. withrespect to the rare-earth alloy, may be used.

The average grain size D of the abrasive grains preferably satisfies 20μm≦D≦60 μm.

The core wire preferably has a diameter of about 0.15 mm to about 0.2mm.

In a preferred embodiment, the step of moving the wire saw includes thestep of moving the wire saw on a plurality of rollers. Each of theplurality of rollers preferably includes a polymer layer on which aguide groove is provided, the guide groove has a pair of slopedsurfaces, at least one of which defines an angle of about 25 degrees toless than about 45 degrees with respect to a radial direction of theroller, and the wire is passed between the sloped surfaces.

The resin layer is preferably made of a phenol resin, an epoxy resin ora polyimide resin.

In a preferred embodiment, the rare-earth alloy is an R—Fe—B basedrare-earth sintered alloy and may be an Nd—Fe—B based rare-earthsintered alloy.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view showing how to define anabrasive grain protruding ratio for a wire saw, and FIG. 1B is aschematic cross-sectional view showing how to define a thicknessdeviation percentage for a wire saw.

FIG. 2 is a schematic representation illustrating a wire saw machine,which can be used effectively to carry out a rare-earth alloy cuttingmethod according to a preferred embodiment of the present invention.

FIG. 3 is a schematic representation illustrating a configuration forthe wire saw machine shown in FIGS. 1A and 1B in the vicinity of themachined portion.

FIG. 4 schematically illustrates a cross-sectional structure for thewire saw that can be used effectively to carry out the rare-earth alloycutting method of a preferred embodiment of the present invention.

FIGS. 5A and 5B schematically illustrate the distributions of abrasivegrains on the wire saw that can be used effectively to carry out therare-earth alloy cutting method of a preferred embodiment of the presentinvention wherein FIG. 5A shows a situation where the average distance Lbetween two adjacent abrasive grains is about 200% of the average grainsize of the abrasive grains, and FIG. 5B shows a situation where theaverage distance L is about 300% of the average grain size.

FIG. 6 schematically illustrates a cross-sectional structures forrollers that are preferably used in the wire saw machines of preferredembodiments of the present invention.

FIG. 7 schematically illustrates a cross-sectional structure for aconventional roller.

FIG. 8 is a graph showing relationships between the tilt angle of theslope of the roller guide groove and the wire saw torsional angle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A rare-earth alloy cutting method according to a preferred embodiment ofthe present invention uses a wire saw obtained by fixing abrasive grains(which are preferably diamond abrasive grains) on a core wire (which ispreferably a piano wire) with a resin layer. The method includes thestep of moving the wire saw while a portion of the rare-earth alloybeing machined with the wire saw is immersed in a coolant, whichpreferably is mainly composed of water and which preferably has asurface tension of about 25 mN/m to about 60 mN/m at about 25° C.,thereby cutting the rare-earth alloy. In the wire saw, an averagedistance between two of the abrasive grains, which are adjacent to eachother in a length direction (i.e., the wire saw traveling direction), ispreferably about 150% to less than about 400% of the average grain sizeof the abrasive grains, an average height of portions of the abrasivegrains, protruding from the surface of the resin layer, is preferablyabout 70% or less of the average grain size of the abrasive grains, anda thickness deviation percentage of the resin layer with respect to thecore wire is preferably about 40%. Optionally, a coolant having akinetic friction coefficient of about 0.1 to about 0.3 with respect tothe rare-earth alloy at about 25° C. may also be used as the coolant.

In the rare-earth alloy cutting method of this preferred embodiment ofthe present invention, the process step of cutting a rare-earth alloywith a fixed abrasive wire saw is carried out with its machined portionimmersed in a coolant having a surface tension of about 25 mN/m to about60 mN/m (i.e., about 25 dyn/cm to about 60 dyn/cm) at about 25° C. Thus,the wire saw can be cooled efficiently. A coolant having a surfacetension that falls within this range exhibits higher wettability (i.e.,conformability) with respect to the rare-earth alloy and/or wire sawthan water does. For that reason, the coolant would penetrate themachined portion (i.e., the portion where the rare-earth alloy and wiresaw contact each other, where the rare-earth alloy is being machined andwhich will be also referred to herein as a “machined groove”)efficiently enough. The coolant mainly composed of water naturally has ahigher specific heat than an oil coolant (e.g., a mineral oil), andtherefore, achieves higher cooling efficiency. As used herein, the“coolant mainly composed of water” refers to a coolant, of which atleast approximately 70 wt % is water.

A preferred coolant, which is used effectively in the rare-earth alloycutting method of preferred embodiments of the present invention, canalso be determined by its kinetic friction coefficient with respect tothe rare-earth alloy. Specifically, a coolant having a kinetic frictioncoefficient of about 0.1 to about 0.3 at approximately 25° C. achievesthe same functions and effects as a coolant having a surface tensionthat falls within the range specified above. The surface tension isregarded as an index to the permeability of the coolant with respect tothe machined portion. On the other hand, the kinetic frictioncoefficient is regarded as an index to the degree of lubricity producedby the coolant in the machined portion. It is also known that there is aqualitative correlation between the surface tension and the kineticfriction coefficient.

The surface tension of the coolant is measured with a well-known DuNouyTensiometer. Also, the kinetic friction coefficient of the coolant withrespect to the rare-earth alloy is measured with a Masuda's “Four-ballFriction Tester”, which is often used as a basic tester in Japan. Thevalues of both the surface tension and kinetic friction coefficient atabout 25° C. are preferably adopted herein as parameters characterizingthe coolant.

It should be noted that the kinetic friction coefficients that will becited in the specific examples to be described later were obtained witha four-ball friction tester including iron balls. The R—Fe—B basedrare-earth alloy to be adopted as an example (where R is one of therare-earth elements including Y and which is an alloy including anNd₂Fe₁₄B intermetallic compound as a main phase, for example) includesiron at a higher percentage than any other constituent element thereof.Accordingly, the kinetic friction coefficient of the coolant, obtainedwith the iron balls, should be a well-approximated kinetic frictioncoefficient with respect to the rare-earth alloy. The present inventorsactually confirmed this point via experiments. The composition andmanufacturing process of a rare-earth alloy that can be used effectivelyas a material for a rare-earth magnet are disclosed in U.S. Pat. Nos.4,770,723 and 4,792,368, for example. In a typical composition of anR—Fe—B based rare-earth alloy, Nd or Pr is often used as R, a portion ofFe may be replaced with a transition element (such as Co), and a portionof B may be replaced with C.

The coolant for use in the cutting process of preferred embodiments ofthe present invention is determined by either the surface tension or thekinetic friction coefficient at approximately 25° C. However, thetemperature of the coolant is not always about 25° C. during its actualuse. Nevertheless, to achieve the effects and advantages of preferredembodiments of the present invention, the temperature of the coolant tobe used is preferably controlled to fall within the range of about 15°C. to about 35° C., more preferably within the range of about 20° C. toabout 30° C., and even more preferably within the range of about 23° C.to about 28° C. As is well known in the art, the surface tension andkinetic friction coefficient of a coolant depend on the temperature.Accordingly, if the temperature of the coolant actually used deviatesfrom these preferred temperature ranges by a large amount, then thesurface tension and kinetic friction coefficient of the coolant wouldalso substantially deviate from their preferred ranges. As a result, thecooling efficiency or cutting efficiency would decline significantly.

The coolant may be prepared by adding either a surfactant or a syntheticlubricant (i.e., a so-called “synthetic”) to water. A predeterminedsurface tension or kinetic friction coefficient is achievable bychanging the type or the amount of the additive appropriately. Inaddition, if such a coolant mainly composed of water is used, then thecutting debris of the rare-earth alloy can be easily separated from thecutting-created sludge with a magnet and the coolant can be recycledbecause the coolant has a relatively low viscosity. It is also possibleto prevent the coolant waste disposal process from having harmfuleffects on natural environments. Furthermore, the amount of carbonincluded in the sludge can be reduced. As a result, the magnet to bemade of the cutting debris collected from the sludge can have improvedmagnetic properties.

If the workpiece is cut with a wire saw traveling at a high velocity,then the coolant may foam to decrease the cooling efficiencyunintentionally. However, by using a coolant including an antifoamingagent, such decreases in cooling efficiency due to foaming of thecoolant can be minimized. Furthermore, when a coolant with a PH of about8 to about 11 is used, the corrosion of the rare-earth alloy can beminimized. More preferably, a coolant with a PH of about 9 or more ispreferably used. Moreover, by using a coolant including a rustpreventive material, the oxidation of the rare-earth alloy can bereduced significantly. The amounts of these additives may beappropriately adjusted with the type and cutting conditions of therare-earth alloy and other factors taken into account.

A wire saw, on which diamond abrasive grains are fixed with a resinlayer, is preferably used as the wire saw. More specifically, a wiresaw, obtained by fixing diamond abrasive grains on the outer surface ofa core wire (which is preferably a piano wire) with a resin, ispreferably used. Among other things, a phenol resin, an epoxy resin anda polyimide resin are particularly preferred. The reasons are asfollows. First, these resins show not only high bond strengths withrespect to the outer surface of the piano wire (i.e., hard-drawn wire)but also excellent wettability (i.e., permeability) with respect to thecoolant as will be described later. If necessary, a filler (such as SiCor Al₂O₃) may be added to the resin (see Japanese Patent Publication No.3078020, for example). Secondly, the wire saw on which abrasive grainsare fixed with a resin layer is less expensive than a wire saw to beproduced by an electrodeposition process, thus reducing the cost ofcutting a rare-earth alloy. It should be noted that the core wire of thewire saw does not have to be a piano wire but may also be made of anNi—Cr alloy, an Fe—Ni alloy or any other suitable alloy, W, Mo or anyother suitable refractory metal, or a bundle of high-strength fiberssuch as nylon fibers. Also, the material of the abrasive grains is notlimited to diamond, either, but may be SiC, B, C or CBN (cubic boronnitride), for example.

By using the coolant described above, the abnormal increase in thetemperature of the wire saw during the cutting process can be reducedsignificantly compared with the situation where water is used as thecoolant. Thus, derailing of the abrasive grains, peeling of the resinlayer and snapping of the wire saw can be minimized. In the rare-earthalloy cutting method of this preferred embodiment of the presentinvention, not only the coolant but also the wire saw are selectedappropriately, thus making it possible to use the wire saw for even alonger time and further reduction of the manufacturing cost. As will bedescribed later by way of experimental examples, the abnormal derailing,resin peeling and snapping can be minimized by using a wire saw in whichan average distance between two abrasive grains, which are adjacent toeach other in the length direction (i.e., the traveling direction), ispreferably about 150% to less than about 400% of the average grain sizeD of the abrasive grains, an average height of portions of the abrasivegrains, protruding from the surface of the resin layer, is preferablyabout 70% or less of the average grain size D of the abrasive grains,and a thickness deviation percentage of the resin layer with respect tothe core wire is preferably about 40%.

As will be described later by way of experimental examples, the presentinventors presume, based on the results of extensive research, that byadjusting not only the density of the abrasive grains, fixed on theouter surface of the core wire with the resin layer, as measured in thelength direction of the wire saw and the average height of the portionsof the abrasive grains protruding from the resin layer (which will bereferred to herein as a “protrusion percentage”) but also the thicknessdeviation percentage of the resin layer with respect to the core wire,the cutting debris (or sludge) flushing capability could be maintainedwithin a good range and the load to be placed on the respective abrasivegrains during the cutting process would become uniform, thus minimizingthe abnormal derailing, resin peeling and snapping.

Hereinafter, the protrusion percentage Pr and thickness deviationpercentage Td, characterizing the wire saw to be used in the cuttingmethod of this preferred embodiment of the present invention, will bedefined with reference to FIGS. 1A and 1B.

In the wire saw 20 shown in FIG. 1A, abrasive grains 24 are fixed on theouter surface of a core wire 22 with a resin layer 26. Supposing theaverage grain size of the abrasive grains 24 is D and the height of theportions of the abrasive grains 24 protruding from the resin layer 26(i.e., the protrusion height) is P, the protrusion percentage Pr (%) isdefined as the ratio of the protrusion height P to the average grainsize D. That is to say, Pr=(P/D)×100. The protrusion percentage Pr canbe obtained from optical micrographs showing the cross sections of thewire saw 20, for example.

As disclosed in Japanese Patent Publication No. 3078020, the wire saw 20may be made by coating the outer surface of a core wire with an abrasivegrain dispersed resin (to which a solvent is added as needed) by amethod called the “enamel method”, for example. In this case, unless thethickness of the resin layer is adjusted sufficiently, the thickness ofthe resin layer 26 may become non-uniform with respect to the core wire22 on a cross section including the diameter of the wire saw 20 (or corewire 22) as schematically shown in FIG. 1B. To evaluate such a deviationin the thickness of the resin layer 26 quantitatively, the thicknessdeviation percentage Td (%) is defined as follows.

Supposing the radius of the core wire 22 is Rw, the minimum thickness ofthe resin layer 26 is Trl and the maximum thickness of the resin layer26 is Trh, the thickness deviation percentage Td is given by(Trh−Trl)/{(Trh+Trl)/2}. Optionally, the denominator (Trh+Trl)/2 may beregarded as the average thickness of the resin layer 26. The thicknessdeviation percentage Td is obtained by actually measuring the radius Rwof the core wire 22 and the minimum and maximum thicknesses Trl and Trhof the resin layer 26. The thicknesses of the resin layer 26 can beobtained from optical micrographs showing the cross sections of the wiresaw 20, for example. Naturally, the ideal thickness deviation percentageof the resin layer 26 is 0%.

Also, as disclosed in Japanese Patent Publication No. 3078020, a wiresaw 20 satisfying the conditions described above can be obtained bycontrolling the thickness of the resin layer with a floating dice in theprocess step of applying the resin by the enamel method. Such a wire saw20 can be supplied from a general wire saw manufacturer (e.g., AlliedMaterial Corp.) if the specifications described above are designatedclearly.

To achieve the effect of small machining allowance, the wire saw 20preferably has an outside diameter of about 0.3 mm or less, morepreferably about 0.25 mm or less. The lower limit of the outsidediameter of the wire saw 20 is defined so as to achieve sufficientstrength. In addition, to fix abrasive grains of a predetermined sizewith sufficient strength, a core wire 22 with a diameter of about 0.12mm to about 0.20 mm, more preferably about 0.15 mm to about 0.20 mm, ispreferably used.

In view of the cutting efficiency, the average grain size D of theabrasive grains 24 preferably satisfies 20 μm≦D≦60 μm, more preferablysatisfies 30 μm≦D≦60 μm, and most preferably satisfies 40 μm≦D≦60 μm.

By using a wire saw 20 according to a preferred embodiment of thepresent invention, good cutting efficiency and debris flushingcapability are achieved. Thus, a workpiece can be machined even if thewire saw is allowed to travel at a relatively high velocity (e.g., at1,000 m/min) and the wire saw can be used for a longer time than theconventional one. In addition, since the rare-earth alloy can be cooledefficiently with the coolant described above, the rare-earth alloy canbe constantly machined for a long time with good machining accuracy.When the coolant mainly composed of water is used, the cuttingefficiency can be optimized by setting the traveling velocity at about20-30% higher (e.g., in the range of about 1,100 m/min to about 1,200m/min) than the situation where an oil coolant is used.

The coolant mainly composed of water for use in the cutting method ofpreferred embodiments of the present invention preferably has lowviscosity (with a kinematic viscosity of about 1 mm²/s). Accordingly,the resultant cutting debris flushing capability is lower than thatachieved by an oil coolant (which usually has a kinematic viscosity of 5mm²/s or more). Thus, in order to increase the cutting debris flushingcapability, the machined portion is preferably kept immersed in thecoolant stored in the vessel in the cutting process. In addition, thecoolant is preferably supplied into the vessel not only through itsbottom but also from its opening such that the coolant is alwaysoverflowing from the opening of the vessel.

The cutting debris, which has been flushed into the low-viscositycoolant, easily precipitates and just a small amount of cutting debrisis floating around the opening of the vessel. To cut the workpiece withthe machined portion immersed in the coolant, the wire saw needs to bearranged so as to travel through the coolant in the vicinity of theopening of the vessel. Accordingly, the wire saw travels in the coolantincluding little cutting debris, and the machined portion is suppliedwith the coolant including little cutting debris. In particular, if thecoolant is also supplied from the vessel opening so as to be alwaysoverflowing from the opening, the coolant supplied to the machinedportion can have a decreased amount of cutting debris. Furthermore, thecutting debris that has been deposited on the wire saw can also bewashed away mechanically with the flow of the coolant supplied from thevessel opening. The quantity of the coolant overflowing per minute ispreferably about 50% or more of the volume of the vessel. Also, theamount of the coolant supplied from the opening is preferably greaterthan that of the coolant supplied through the bottom of the vessel.

Furthermore, if a curtain-like flow of the coolant (or the air) isformed over the four sides of the vessel opening intersecting with thewire saw traveling direction and if the surface of the overflowingcoolant is kept higher than the vessel walls by reducing the amount ofthe coolant overflowing from the vessel opening, then an even greaterquantity of coolant is supplied to the area around the machined portion.As a result, the amount of the cutting debris in the coolant can befurther decreased. A discharge pressure to form the coolant flow ispreferably in the range of about 0.2 MPa (i.e., about 2 kgf/cm²) toabout 1.0 MPa (i.e., about 10 kgf/cm²), more preferably in the range ofabout 0.4 MPa (i.e., about 4 kgf/cm²) to about 0.6 MPa (i.e., about 6kgf/cm²). The reasons are as follows. Specifically, if the dischargepressure is lower than these ranges, sufficient effects may not beachieved. However, if the discharge pressure exceeds these ranges, thenthe wire saw flexes so much that the machining accuracy may decrease.

Also, among a plurality of main rollers provided to make the wire sawtravel thereon, two main rollers, arranged on both sides of the vesselto regulate the traveling path of the wire saw, are also preferablysupplied with a coolant that has been discharged. By discharging thecoolant to these main rollers, the increase in the temperature of apolymer layer (e.g., an organic polymer layer such as a urethane rubberlayer), which is provided on the surface of the main rollers so as tohave a wire saw guide groove, can be minimized. In addition, the cuttingdebris (or sludge) that has been deposited or left either on the wiresaw or in the guide groove can also be washed away, thereby preventingthe wire saw traveling path from deviating or the wire saw fromderailing from the groove.

Also, by collecting a dirty liquid, including the sludge with therare-earth alloy cutting debris produced by the cutting process and thecoolants, and by getting the rare-earth alloy cutting debris separatedout from the sludge by a magnet, the coolants can be recycled (e.g.,used cyclically). As described above, since the coolant mainly composedof water has low viscosity, the cutting debris can be easily sorted outtherefrom. Also, by sorting out the rare-earth alloy cutting debris, thecoolant waste can be disposed of easily without doing any damage on theenvironment. Furthermore, carbon, which is not easily separable from anR—Fe—B based alloy (where R is one of the rare-earth elements thatinclude Y), can be reduced and the cutting debris may also be used as arecycling material for a rare-earth alloy. Since the coolant preferablyis mainly composed of water, it is easy to decrease the amount of carbonincluded in the rare-earth alloy that has been recycled from the cuttingdebris. Thus, a raw material that may be used as a material for arare-earth magnet can be obtained. As a method of sorting out thecutting debris from the sludge, the method disclosed by the applicant ofthe present application in Japanese Laid-Open Publication No. 2002-36113may be used.

By adopting the cutting method of preferred embodiments of the presentinvention, a rare-earth alloy can be cut with high accuracy andefficiency. Thus, a small rare-earth magnet (with a thickness of about0.5 mm to about 3.0 mm, for example) for use in a voice coil motor toposition a magnetic head can be produced with high accuracy andefficiency.

Hereinafter, a rare-earth alloy cutting method according to a preferredembodiment of the present invention will be described in further detailwith reference to the accompanying drawings. In the following preferredembodiment, a method of cutting a neodymium magnet sintered body to makethe neodymium magnet mentioned above will be described as an example.

A method for producing a neodymium (Nd—Fe—B based) sintered magnet willbe described briefly. It should be noted that a method of making arare-earth alloy as a magnet material is disclosed in detail in U.S.Pat. Nos. 4,770,723 and 4,792,368 identified above, for example.

First, material metals are exactly weighed to a predetermined mixtureratio and then melted in a high-frequency melting crucible within avacuum or an argon gas atmosphere. The molten material metals are thenpoured into a water-cooled casting mold, thereby obtaining a materialalloy with a predetermined composition. Then, this material alloy ispulverized to make a fine powder with an average grain size of about 3μm to about 4 μm. Subsequently, this fine powder is introduced into adie and then pressed and compacted under a magnetic field. If necessary,this compacting process step is carried out after the fine powder hasbeen mixed with a lubricant. Next, the compact is subjected to asintering process at a temperature of about 1,000° C. to about 1,200°C., thereby obtaining a neodymium magnet sintered body. Thereafter, toincrease the coercivity of the magnet, the sintered body is furthersubjected to an aging treatment at about 600° C. to complete themanufacturing process of the rare-earth magnet sintered body. Thesintered body may have dimensions of approximately 30 mm×50 mm×50 mm,for example.

Then, the resultant sintered body is machined and cut, thereby dividingthe sintered body into multiple thin plates (which will be sometimesreferred to herein as “substrates” or “wafers”). Next, each of thesethin plates of the sintered body is polished and finished to adjust itsshape and dimensions. Subsequently, to improve the long-termreliability, the thin plate is further subjected to a surface treatment.Thereafter, the thin plate further goes through a magnetizing processand a testing process, thereby completing a neodymium permanent magnet.Alternatively, the magnetizing process may be carried out before thecutting process.

Hereinafter, a cutting process according to a preferred embodiment ofthe present invention will be described in further detail with referenceto FIGS. 2 through 5B.

FIG. 2 is a schematic representation illustrating a wire saw machine100, which can be used effectively to carry out a rare-earth alloycutting process according to a preferred embodiment of the presentinvention.

The wire saw machine 100 includes three main rollers 10 a, 10 b and 10 cand two reel bobbins 40 a and 40 b. The main roller 10 a, provided undera vessel 30 containing a coolant, is the drive roller, while the othermain rollers 10 b and 10 c, arranged on both sides of the vessel 30, aredriven rollers. The wire saw 20 travels back and forth while beingreeled off one of the two reel bobbins (e.g., 40 a) and reeled up in theother reel bobbin (e.g., 40 b). That is to say, the wire saw 20 ispreferably driven by a so-called “reciprocating drive method”. In thiscase, if the reeling time of the reel bobbin 40 a is longer than that ofthe other reel bobbin 40 b, then a brand new wire saw 20 can be fed tothe reel bobbin 40 a while allowing the wire saw 20 to move back andforth. The wire saw 20 preferably travels at a velocity of about 600m/min to about 1,500 m/min. And the new wire may be fed at a rate ofabout 1 m/min to about,5 m/min, for example.

The wire saw 20 may be strung in 150 lines, for example, between themain rollers 10 a, 10 b and 10 c. To define the traveling path of thewire saw 20, a polymer layer (e.g., an organic polymer layer such as aurethane rubber layer) with a guide groove (having a depth of about 0.6mm, for example, and not shown) for guiding the wire saw 20 is providedon the surface of the main rollers 10 a, 10 b and 10 c. The line-to-linespacing of the wire saw 20 is defined by the pitch of this guide groove.And the guide groove pitch is adjusted according to the thickness ofplates to be cut out from the workpiece. As the polymer layer, aninorganic polymer layer made of a silicone elastomer may also be used.

Traversers 42 a and 42 b for adjusting the reeling positions areprovided in the vicinity of the reel bobbins 40 a and 40 b,respectively. Along the path leading from the reel bobbin 40 a or 40 bto the main roller 10 a, five guide rollers 44 and one tension roller 46are preferably provided on each side, thereby guiding the wire saw 20and regulating its tension. The tension of the wire saw 20 may beappropriately changed according to a combination of various conditionsincluding cut length, cutting speed and traveling velocity. For example,the wire 20 may have a tension of about 20 N to about 40 N.

The sintered body workpieces 50 obtained as described above are set inthis wire saw machine 100 in the following manner.

A number of workpieces 50 are bonded together with an epoxy adhesive(not shown), for example. After these workpieces 50 have been assembledinto a plurality of blocks, these blocks are fixed onto a ferrous workplate 54 with carbon base plates 52 interposed between them. The workplate 54, blocks of workpieces 50 and carbon base plates 52 are alsobonded together with an adhesive (not shown). The carbon base plate 52is subjected to the machining by the wire saw 20 after the workpieces 50have been cut and until the work plate 54 stops descending. In thismanner, the carbon base plates 52 function as a dummy for protecting thework plate 54.

In this preferred embodiment, each block is designed so as to have asize of about 100 mm as measured in the direction in which the wire saw20 travels. Although the workpieces 50 are arranged as a plurality ofblocks in the preferred embodiment described above, the size to bedefined in the traveling direction of the wire saw 20 is also changeablewith the surface tension of the coolant or the traveling velocity. Also,the number and arrangement of workpieces 50 that makes up a single blockcan change with the size of each workpiece 50. Accordingly, theworkpieces 50 may be appropriately arranged as blocks of the best sizein view of these considerations.

The workpieces 50, which have been set in this manner, are lowered by anelevator with a motor 58 and pressed against the traveling wire saw 20so as to be cut and machined. The lowering velocity of the workpieces 50is changeable with various conditions but may be within the range ofabout 20 mm/hr to about 50 mm/hr, for example.

The coolant stored in the coolant tank 60 is pumped up by a dischargepump 62 and transferred by way of the piping 63. The piping 63 branchesinto a lower pipe 64 and an upper pipe 66, which are provided withvalves 63 b and 63 a, respectively, to control the flow rate of thecoolant. The lower pipe 64 is connected to a lower nozzle 64 a, which isprovided at the bottom of the vessel 30 in order to immerse the machinedportion in the coolant. On the other hand, the upper pipe 66 isconnected to not only upper nozzles 66 a, 66 b and 66 c for supplyingthe coolant from the opening of the vessel 30 but also two more uppernozzles 66 d and 66 e provided for the purpose of cooling the mainrollers 10 b and 10 c, respectively.

The coolant is supplied into the vessel 30 through the upper nozzles 66a, 66 b and 66 c and lower nozzle 64 a. At least during the cuttingprocess, the coolant is kept overflowing from the opening of the vessel30 as indicated by the arrow F in FIG. 2. The coolant that hasoverflowed from the vessel 30 is guided to, and accumulated in, acollecting tank 72 by way of the collecting pan 70 provided under thevessel 30. The collected coolant may be pumped up by a discharge pump 74and returned to the coolant tank 60 by way of a circulating pipe 76 asshown in FIG. 2, for example. A filter 78 is preferably provided atapproximately a midway point of the circulating pipe 76 so as to sortout and remove the cutting debris from the coolant collected. However,the collecting method is not limited to the illustrated one.Alternatively, a mechanism for sorting out and separating the cuttingdebris by magnetic force may be provided (see Japanese Laid-OpenPublication No. 2002-36113, for example).

Hereinafter, a cutting process according to a preferred embodiment ofthe present invention will be described in further detail with referenceto FIG. 3.

The vessel 30 includes auxiliary walls 32 on its sidewalls, intersectingwith the traveling direction of the wire saw 20, and in the vicinity ofits opening. These auxiliary walls 32 may be plastic plates (e.g.,acrylic plates) and are arranged so as to be located near the travelingpath of the non-loaded wire saw as indicated by the dashed line in FIG.3. When the workpieces 50 are lowered and brought into contact with thewire saw 20 so as to be cut, the wire saw 20 flexes and the machinedportions are immersed in the coolant in the vessel 30 as indicated bythe solid curve in FIG. 3. In this case, as the wire saw 20 is bentdownward, the wire saw 20 cuts off the auxiliary walls 32, therebyforming slits. When the cutting by the wire saw 20 reaches a steadystate, the magnitude of the flexure becomes constant. As a result, thewire saw 20 is moving to cut the workpieces 50 while passing the slitsthat have been created through the auxiliary walls 32. In this manner,the slits, formed through the auxiliary walls 32, function so as toregulate the traveling path of the wire saw 20 and contribute tostabilizing the machining accuracy.

The vessel 30 may have a capacity of about 35 L (litters), for example.During the cutting process, the vessel 30 is preferably supplied withthe coolant at a flow rate of about 30 L/min through the lower nozzle 64a and at a flow rate of about 90 L/min through the upper nozzles 66 a,66 b and 66 c, respectively, such that the coolant is always overflowingfrom the opening. In order to supply the coolant to just the wire saw20, it is not always necessary to make the coolant overflow because thewire saw 20 flexes during the cutting process as shown in FIG. 2.However, in cutting a neodymium magnet sintered body as in this example,such a configuration is preferably adopted to increase the cuttingdebris flushing capability.

To increase the cutting debris flushing capability, it is effective toreduce the amount of the cutting debris included in the coolant in thevicinity of the machined portions. To achieve sufficient flushingcapability, the quantity of the coolant overflowing per minute ispreferably about 50% or more of the vessel volume. Furthermore, agreater quantity of fresh coolant is preferably supplied from theopening of the vessel 30 rather than through the bottom thereof. In thispreferred embodiment, a low-viscosity coolant mainly composed of wateris preferably used. Accordingly, the cutting debris that has beenflushed away into the coolant precipitates easily. For that reason, if alot of coolant is supplied through the bottom of the vessel 30, theprecipitated cutting debris may be floating around the machined portionsunintentionally.

Also, the percentage of the fresh coolant to be supplied from theopening is preferably increased. That is to say, by additionallysupplying the coolant from the opening of the vessel 30 and keeping thecoolant overflowing from the opening, the amount of the cutting debrisincluded in the coolant being supplied to the machined portions can bereduced. Furthermore, the cutting debris that has been deposited on thewire saw 20 can also be washed away mechanically with the flow of thecoolant supplied from the opening of the vessel 30.

Also, the auxiliary walls 32 described above function as the sidewallsof the vessel 30 except for the slits that have been formed by the wiresaw 20, and therefore, can contribute to keeping the surface S of thecoolant high. Furthermore, if a curtain-like coolant flow is formedaround the opening of the vessel 30 with the nozzles 66 b and 66 e so asto intersect with the traveling direction of the wire saw 20 and if thesurface S of the overflowing coolant is kept higher than the auxiliarywalls 32 of the vessel 30 by reducing the amount of the coolantoverflowing from the opening of the vessel 30, then an even greaterquantity of coolant is supplied to the area around the machinedportions. As a result, the amount of the cutting debris in the coolantcan be further decreased. A discharge pressure to form the coolant flowis preferably in the range of about 0.2 MPa (i.e., about 2 kgf/cm²) toabout 1.0 MPa (i.e., about 10 kgf/cm²), more preferably in the range ofabout 0.4 MPa (i.e., about 4 kgf/cm²) to about 0.6 MPa (i.e., about 6kgf/cm²). The reasons are as follows. Specifically, if the dischargepressure is lower than these ranges, sufficient effects may not beachieved. However, if the discharge pressure exceeds these ranges, thenthe wire saw 20 may flex so much that the machining accuracy maydecrease.

Also, the two main rollers 10 b and 10 c, arranged on both sides of thevessel 30 to regulate the traveling path of the wire saw 20, are alsopreferably supplied with the coolant that has been discharged. Bydischarging the coolant to these main rollers 10 b and 10 c, theincrease in the temperature of a polymer layer (e.g., a urethane rubberlayer), which is provided on the surface of the main rollers 10 b and 10c so as to have a guide groove for the wire saw 20, can be minimized. Inaddition, the cutting debris (or sludge) that has been deposited or lefteither on the wire saw 20 or in the guide groove can also be washedaway, thereby preventing the traveling path of the wire saw 20 fromdeviating or the wire saw 20 from derailing from the groove.

Examples of the surfactants to be added to the coolant mainly composedof water include anionic surfactants and nonionic surfactants. Preferredanionic surfactants include fatty acid derivatives including fatty acidsoap and naphthenic acid soap, ester sulfates such as a long-chainalcohol ester sulfate and an oil sulfate (e.g., an animal or vegetableoil sulfate), and sulfonic acids such as petroleum sulfonates. Preferrednonionic surfactants include polyoxy-ethylenes such aspolyoxyethylenealkylphenyl ether and polyoxyethylene mono fatty acidester, polyhydric alcohols such as sorbitan mono fatty acid ester, andalkylol amides such as fatty acid diethanol amide. Specifically, byadding about 2 wt % of water to a chemical solution type JP-0497N(produced by Castrol Limited), the kinetic friction coefficient can beadjusted so as to fall within the predetermined range.

Also, examples of preferred synthetic lubricants include syntheticsolution types, synthetic emulsion types and synthetic soluble types.Among other things, synthetic solution types are particularly preferred.More specifically, Syntairo 9954 (produced by Castrol Limited) and #830or #870 (produced by Yushiro Chemical Industry Co., Ltd.) may be used.In any case, by adding about 2 wt % to about 10 wt % of such a lubricantto water, the surface tension (or kinetic friction coefficient) can becontrolled to be within a preferred range.

Furthermore, by adding a rust preventive material, corrosion of therare-earth alloy can be prevented. In particular, in cutting an R—Fe—Bbased rare-earth alloy, PH is preferably set somewhere between about 8and about 11, more preferably at least equal to about 9. Examples ofrust preventive materials include organic ones and inorganic ones.Preferred organic rust preventive materials include carboxylates such asoleates and benzoates and amines such as triethanolamine. Preferredinorganic rust preventive materials include phosphates, borates,molybdates, tungstates and carbonates.

Also, a nitrogen compound such as benztriazol may be used as anonferrous metal anticorrosive agent. A formaldehyde donor such ashexahydrotriazine may be used as a preservative.

A silicone emulsion may be used as an antifoaming agent. By adding theantifoaming agent, foaming of the coolant can be reduced, thepermeability and cooling effect of the coolant can be improved, and theabnormal temperature increase or abnormal abrasion of the wire saw 20can be minimized.

Next, the structure of the wire saw 20 to be preferably used in thispreferred embodiment will be described with reference to FIGS. 4, 5A and5B. It should be noted that the structure of the lower half of the wiresaw 20 below the one-dot chain is simplified in FIG. 4.

A wire saw, obtained by fixing diamond abrasive grains 24 on the outersurface of a core wire (e.g., a piano wire) 22 with a resin layer 26, ispreferably used as the wire saw 20. Among other things, a phenol resin,an epoxy resin and a polyimide resin are particularly preferred. This isbecause these resins show not only high bond strengths with respect tothe outer surface of the piano wire (i.e., hard-drawn wire) 22 but alsoexcellent wettability (i.e., permeability) with respect to the coolantdescribed above.

More specifically, a preferred wire saw 20 having an outside diameter ofabout 0.24 mm may be obtained by fixing diamond abrasive grains, havingan average grain size of about 40 μm, on the outer surface of a pianowire 22, having a diameter of about 0.18 mm, with a phenol resin layer26, for example. Also, considering the cutting efficiency and thecutting debris (sludge) flushing efficiency, the average distancebetween two adjacent abrasive grains 26 in the length direction of thewire saw 20 (i.e., the axial direction that is parallel to the one-dotchain in FIG. 4) is preferably about 150% to less than about 400% of theaverage grain size D of the abrasive grains.

For example, if the average distance L between two abrasive grains,which are adjacent to each other in the length direction of the wire saw20, is about 200% or about 300% of the average grain size D of theabrasive grains as shown in FIGS. 5A and 5B, then the load placed on therespective abrasive grains 24 is reduced. As a result, the abnormalderailing of the abrasive grains 24, peeling of the resin layer 26 andsnapping are minimized. In other words, if the average distance Lbetween adjacent abrasive grains becomes about 400% or more, then thedistribution density of the abrasive grains 24 becomes too low. Thus,the load to be placed on the respective abrasive grains 24 during thecutting process becomes so heavy as to cause that abnormal derailing.However, if the average distance L between adjacent abrasive grainsbecomes less than about 150%, then the distribution density of theabrasive grains 24 becomes too high. Accordingly, the chip pocketcapacity to be described later is insufficient and the cutting debrisflushing capability decreases. As a result, the cutting efficiencydecreases, too.

FIGS. 5A and 5B schematically illustrate uniform distributions of theabrasive grains 24 by extending the outer surface of the wire saw 20(with a length of about 1.6 mm, for example) into plan views. Actually,though, the abrasive grains 24 are distributed non-uniformly. However,the semi-quantitative effects of the difference in average distance Lbetween adjacent abrasive grains on the distribution density of theabrasive grains can be understood from these drawings. The averagedistance L between adjacent abrasive grains on the wire saw 20 can beactually obtained using optical micrographs, for example.

Furthermore, the protrusion percentage of the abrasive grains 22 on thewire saw 20 is preferably about 70% or less. This is because if theprotrusion percentage exceeds about 70%, then the load placed on theabrasive grains 22 cannot be supported by the resin layer 26sufficiently so as to avoid abnormal derailing and resin layer peeling.Also, considering the cutting debris flushing capability, the protrusionpercentage of the abrasive grains preferably exceeds about 40%. This isbecause if the protrusion percentage is about 40% or less, then thespace 28 between the abrasive grains 22. (i.e., the chip pocket) hassuch a small capacity as to decrease not only the cutting debrisflushing capability but also cutting efficiency often. The size of thechip pocket 28 depends on the distance between adjacent abrasive grainsdescribed above.

Furthermore, in the cutting method of this preferred embodiment, a wiresaw 20, of which the resin layer 26 has a thickness deviation percentageof about 40%, is preferably used. The reason is as follows.Specifically, if the thickness deviation percentage exceeds about 40%,then the load being placed on the resin layer 26, on which the abrasivegrains 24 are fixed, becomes non-uniform. Accordingly, if the cuttingprocess is carried out with such a wire saw 20, then the strength of theresin layer 26 will be locally insufficient, thereby derailing theabrasive grains 24 and peeling off the resin layer 26 easily.

Hereinafter, the relationships between the average distance L betweenadjacent abrasive grains, the protrusion percentage and thicknessdeviation percentage of the abrasive grains on the wire saw 20 and thepeeling and snapping of the resin layer 26, and the profile irregularity(i.e., winding) of the cut surface will be described by way ofexperimental examples.

In the following experimental examples, a sintered block of a neodymiummagnet (having a length of about 40 mm in the traveling direction, alateral length of about 50 mm and a thickness of about 30 mm, forexample) was cut with the wire saw machine 100 shown in FIG. 2 by themethod described above so as to divide the lateral sides of the block.As the coolant, a coolant, of which the surface tension and kineticfriction coefficient were adjusted to about 34.6 mN/m and about 0.13,respectively, by adding WS-250B (produced by Yushiro Chemical IndustryCo., Ltd.) to tap water, was used. The wire saw 20 was allowed to travelat a velocity of about 1,100 m/min and at a cutting speed of about 40mm/hr (in the thickness direction).

The core wire (e.g., piano wire) 22 of the wire saw 20 had a diameter ofabout 0.18 mm. The abrasive grains (e.g., diamond abrasive grains) 24had an average grain size of about 42 μm. A phenol resin was used as theresin layer 26, which had an average thickness (i.e., ideal thickness)of about 20 μm. The average distance L between adjacent abrasive grainsand the protrusion percentage and thickness deviation percentage of theabrasive grains on the wire saw 20 were obtained using opticalmicrographs. It should be noted that the thickness deviation percentagewas obtained based on the measuring results of about 10 cross sectionsthat were spaced apart from each other at an interval of about 500 mm.The peeling of the resin layer 26 was evaluated by viewing theappearance of the wire saw 20, which had been used to process a sinteredblock of a neodymium magnet for four hours, with the eyes. For example,if the wire saw 20 had an overall length of about 200 m and if the sumof the lengths of peeled portions (each having a length of at leastabout 5 mm) was about 10 m or more, then the resin layer was regarded as“peeled”. Specifically, if the sum of the lengths of peeled portions wasabout 10 m to about 60 m, then the resin layer was regarded as “peeledslightly” And if the sum exceeded about 60 m, then the resin layer wasregarded as “peeled all over”. Also, if the “slightly peeled” resinlayer had an interval of about 20 m or more between adjacent peeledportions, then the resin layer was regarded as “peeled intermittently”.Furthermore, the profile irregularity of the cut surface was alsomeasured with a contact-type roughness meter, and the maximum windingvalue within a width of about 25 mm was adopted as a representativevalue.

The present inventors analyzed the effects of the average distance Lbetween adjacent abrasive grains in the length direction of the wire saw20. The results are shown in the following Table 1.

As is clear from Table 1, if the average distance L between adjacentabrasive grains was within the range of about 150% to less than about400% of the average grain size of the abrasive grains, no peeling wasproduced in the resin layer 26 and the cut surface also had as small aprofile irregularity as less than about 8 μm. In contrast, if theaverage distance L between adjacent abrasive grains was less than about150% of the average grain size, then the cutting efficiency was too lowto achieve a cutting speed of about 40 mm/hr easily. Also, if theaverage distance L between adjacent abrasive grains became about 400% ormore of the average grain size, the resin layer 26 peeled. And if theaverage distance L reached about 600% of the average grain size, theresin layer 26 peeled all over the wire saw 20 and sometimes the wiresaw 20 snapped. Furthermore, if the average distance L between adjacentabrasive grains was about 400% or more of the average grain size, theprofile irregularity of the cut surface was as large as 8 m or more.Taking these results into consideration, it can be seen that by settingthe average distance L between adjacent abrasive grains on the wire saw20 within the range of about 150% to less than about 400% of the averagegrain size, the life of the wire saw 20 can be extended and a sufficientprofile irregularity is achieved for the cut surface. TABLE 1 Average 4242 42 42 42 42 42 42 42 abrasive grain size D (μm) Average 63 71.4 84105 126 168 210 252 294 grain-to- grain distance L (μm) (L/D) × 100 150170 200 250 300 400 500 600 700 (%) Cut surface 4.2 3.5 3.5 3.2 4.2 8 1012 — profile irregularity (μm) Resin layer No No No No No Yes, Yes, Yes,Snapped peeled? slightly intermittently all over

The present inventors also analyzed the effects of the protrusionpercentage of the abrasive grains 24. The results are shown in thefollowing Table 2.

As can be seen from Table 2, if the wire saw 20 used had a protrusionpercentage of about 71% or more, the resin layer 26 started to peel off.And if the wire saw 20 had a protrusion percentage exceeding about 83%,snapping was produced. Also, if the wire saw 20 adopted had a protrusionpercentage of about 71% or more, the cut surface had a profileirregularity (winding) of about 10 m or more. More preferably, a wiresaw 20 with a protrusion percentage of about 60% or less is used becausethe profile irregularity of the cut surface can be reduced to about 8 mor less. However, if a wire saw with a protrusion percentage of lessthan about 40% was used, no resin layer peeling was produced, asufficient profile irregularity was achieved for the cut surface but thecutting efficiency sometimes decreased. This is why the protrusionpercentage is preferably about 40% or more. TABLE 2 Average 42 42 42 4242 42 42 42 42 abrasive grain size D (μm) Abrasive 0 5 10 15 20 25 30 35<35 grain protrusion height (μm) Abrasive 0 12 24 36 48 60 71 83 <83grain protrusion percentage (%) Cut surface 5.4 3.5 3.5 3.2 4.2 8 10 12— profile irregularity (μm) Resin layer No No No No No No Yes, Yes,Snapped peeled? intermittently all over

The present inventors also analyzed the effects of the thicknessdeviation percentage of the resin layer 26. The results are shown in thefollowing Table 3.

As is clear from Table 3, if the wire saw 20 used had a thicknessdeviation percentage of about 50%, then the resin layer 26 peeled. Andif the wire saw 20 had a thickness deviation percentage of about 100% ormore, snapping was produced. On the other hand, if the wire saw 20 had athickness deviation percentage of less than about 40%, no peeling wasobserved in the resin layer 26 and the cut surface profile irregularitywas as small as about 4 μm or less. More preferably, the thicknessdeviation percentage is about 30% or less. However, even a thicknessdeviation percentage of about 40% or less is sufficientlymass-producible level.

It should be noted that the resin layer 26 of the wire saw 20 may or maynot peel or snap depending on the tension of the wire saw 20 travelingbetween the rollers, too. The results described above were obtained whenthe wire saw 20 had a tension of about 30 N. However, substantially thesame results were obtained even when the wire saw had a tension of about25 N to about 35 N. TABLE 3 Average grain 42 42 42 42 42 42 size D (μm)Ideal 20 20 20 20 20 20 thickness of resin layer (μm) Minimum 20 17 1615 10 5 thickness of resin layer (μm) Maximum 20 23 24 25 30 35thickness of resin layer (μm) Radius (μm) of 90 90 90 90 90 90 core wireThickness 0 30 40 50 100 150 deviation percentage (%) Cut surface 3.5 44 6 12 — profile irregularity (μm) Resin layer No No No Yes, Yes, allSnapped peeled? slightly over

Next, a preferred structure for the main rollers 10 a, 10 b and 10 c ofthe wire saw machine 100 including the wire saw will be described.

If a coolant mainly composed of water is used, the wire saw snappingrate increases (i.e., the wire saw snaps in a shorter time) and themachining accuracy decreases as compared with a situation where an oilcoolant is used. As a result of various experiments, the presentinventors discovered that if the cross-sectional shape of the guidegroove 10G provided on the polymer layer 10P of the rollers 10 a, 10 band 10 c is designed such that the pair of sloped surfaces 10S of theguide groove 10G defines an angle of about 25 degrees to less than about45 degrees (which will be referred to herein as a “tilt angle α”) withrespect to the radial direction 10R of the roller 10 a as schematicallyshown in FIG. 7, the snapping of the wire saw 20 can be further reducedand sufficient machining accuracy is achieved. More preferably, the tiltangle is about 30 degrees to about 35 degrees.

Both of the two sloped surfaces 10S of the guide groove 10G preferablydefine the tilt angle falling within that range with respect to theradial direction R of the roller 10 a as shown in FIG. 7. However, if atleast one of the two sloped surfaces 10S has a tilt angle falling withinthat range, then the wire snapping can be minimized effectively andsufficient machining accuracy is achieved.

In the prior art, a structure in which the sloped surface 10S of theguide groove 10G defines a tilt angle of about 45 degrees or more withrespect to the radial direction R of the roller is adopted as shown inFIG. 7, for example. Such a structure is adopted to remove the sludgesufficiently efficiently from the guide groove 10G. Among other things,a rare-earth alloy includes a main phase causing a brittle fracture anda grain boundary phase causing a ductile fracture, thus producing highcutting resistance. In addition, the rare-earth alloy has such a heavyspecific gravity that it is not easy to remove a sludge including therare-earth alloy. For that reason, to remove the sludge moreefficiently, the tilt angle is preferably greater than about 45 degrees.

However, the present inventors discovered and confirmed via experimentsthat even if the tilt angle of the sloped surface 10S was greater thanabout 45 degrees, the wire snapping rate did not decrease so much butrather the machining accuracy decreased. Hereinafter, this phenomenonwill be described with reference to FIG. 8. As already described withreference to FIG. 2, the wire saw 20 is wound so as to form a pluralityof traveling lines that are parallel to each other between the rollers10 a, 10 b and 10 c. The position of the wire saw 20, which forms thosetraveling lines, is defined by the guide grooves 10G that are providedin the polymer layers 10P of the rollers 10 a, 10 b and 10 c.Accordingly, in passing from one traveling line to its adjacent one, thewire saw 20 is obliquely wound around the guide grooves 10G. Thisobliquely stretched wire saw 20 receives a torsional force from theslopes 10S of the guide grooves 10G. Also, the more obliquely the wiresaw 20 is stretched, the greater the torsional force the wire saw 20receives.

FIG. 8 is a graph showing a relationship between the tilt angle of thesloped surface 10S of the roller guide groove and the wire saw torsionalangle. The torsional angle Ω is proportional to the torsional forceapplied from the roller to the wire saw 20. When the torsional angle Ωis 360 degrees, it is shown that the wire saw has been subjected to onefull torsion. It should be noted that the results shown in FIG. 8 wereobtained by performing dynamic model calculations on the configurationto be described below. Also, the results shown in FIG. 8 were obtainedbased on the assumption that the tilt angles of the two sloped surfaces10S were equal to each other.

Further, 200 lines of the wire saw 20 were stretched at a tension ofabout 30 N (i.e., about 3 kgf) between the pair of rollers (e.g., therollers 10 b and 10 d shown in FIG. 1) having a diameter of about 170mm, which were arranged at a span of about 450 mm. The wire saw 20 wasallowed to travel back and forth in a cycle time of about 120 secondsand at a new wire feed rate of about 2 m/min. In that case, after thewire saw 20 had made 190 rounds, the wire saw got out of the roller.

As a result of various experiments, it was discovered that when the wiresaw was subjected to such torsional force that 5 full torsions (i.e.,Q=1,800 degrees) occurred during one span (about 450 mm), the wire saw20 made about 500 torsions while traveling through the 200 lines. Thatis to say, if the wire saw was subjected to a torsional force thatcaused 1,000 torsions (=200 lines×5 times), approximately 50% of thetorsions were accumulated as actual torsions. Thus, the torsional angleΩ as the ordinate of FIG. 8 was obtained by multiplying a torsionalangle, corresponding to torsional force estimated by the dynamic modelcalculations, by about 0.5. Furthermore, based on the results of astatic torsional fracture strength test, it was estimated that when thetorsional angle actually accumulated in the wire saw 20 reachedapproximately 1,800 degrees (i.e., 5 full torsions), the wire saw shouldcause a fracture at a probability of about 10%.

As can be seen from FIG. 8, as the tilt angle of the groove 10Gincreases, the torsional force (or torsional angle) decreases steadilyand uniformly. Suppose the wire saw 20 causes a fracture simply due tothe torsional force. In that case, if the wire saw 20 is relatively thin(e.g., has a diameter d of about 0.19 mm), then the fracture of the wiresaw 20 can be avoided by setting the tilt angle equal to or greater thanabout 10 degrees. Also, even if the wire saw 20 is relatively thick(e.g., has a diameter d of about 0.25 mm), the fracture of the wire saw20 can be avoided by setting the tilt angle equal to or greater thanabout 25 degrees.

According to the results of experiments, however, no matter whether thewire saw 20 was relatively thin or thick, the wire snapping percentagedid not decrease so much once the tilt angle exceeded about 45 degrees.Also, if the tilt angle was about 45 degrees or more, the machiningaccuracy decreased unintentionally.

The reasons are believed to be as follows. Specifically, as the tiltangle increases, the width 10W of the guide groove 10G (see FIG. 6)increases, thereby allowing the wire saw 20 to swing within the guidegroove 10G or even jump to an adjacent guide groove 10G. Then, thetension or torsional force applied to the wire saw 20 becomesnon-uniform to produce significant stress locally. As a result, the wiresaw 20 snaps. Also, the machining accuracy decreases because the wiresaw 20 could not travel along the groove 10G constantly. The experimentswere carried out by using a urethane rubber layer as the polymer layer10P and an approximately 10% aqueous solution of #830 produced byYushiro Chemical Industry Co., Ltd. as the coolant. Also, rare-earthsintered magnet workpieces were cut as in the experimental exampledescribed above.

Considering these results, the sloped surfaces 10S of the guide groove10G preferably have a tilt angle of about 25 degrees to less than about45 degrees. Also, to minimize the snapping of the wire saw 20, the tiltangle is preferably about 30 degrees or more such that the torsionalforce decreases. To achieve high machining accuracy, the tilt angle ispreferably about 35 degrees or less. Furthermore, the bottom 10B of theguide groove 10G is preferably shaped so as to have a somewhat smallerradius of curvature than the radius of the wire saw 20.

By using such a wire saw machine 100, the wire saw 20, which already hassecured a longer life due to the effects of the preferred embodimentsdescribed above, can have an even longer life. Particularly when arelatively large torsional force is produced (e.g., where theroller-to-roller distance is short), this preferred embodiment achievessignificant effects.

Preferred embodiments of the present invention have been described aspreferably being applied to the wire saw machine 100. However, thepresent invention is in no way limited to those specific preferredembodiments. Alternatively, the present invention is also applicable foruse in an endless wire saw machine using a single reel bobbin (seeJapanese Laid-Open Publication No. 11-198018, for example).

According to the present invention, in a situation where a rare-earthalloy is cut with a wire saw machine using a coolant that is mainlycomposed of water, the life of the wire saw can be extended. Thus, arare-earth sintered alloy to make a rare-earth sintered magnet for usein a voice coil motor can be cut efficiently by using such anenvironmentally friendly coolant mainly composed of water. That is tosay, the manufacturing cost of the rare-earth sintered magnets can bereduced.

1-8. (canceled)
 9. A method of cutting a rare-earth alloy with a wiresaw obtained by fixing abrasive grains on a core wire with a resinlayer, the method comprising the step of moving the wire saw while aportion of the rare-earth alloy being cut by the wire saw is immersed ina coolant, which is mainly composed of water and has a surface tensionof about 25 mN/m to about 60 mN/m at approximately 25° C., therebycutting the rare-earth alloy; wherein in the wire saw, an averagedistance between two of the abrasive grains, which are adjacent to eachother in a length direction, is about 150% to less than about 400% ofthe average grain size of the abrasive grains, an average height ofportions of the abrasive grains, protruding from the surface of theresin layer, is about 70% or less of the average grain size of theabrasive grains, and a thickness deviation percentage of the resin layerwith respect to the core wire is about 40%.
 10. The rare-earth alloycutting method of claim 9, wherein the average grain size D of theabrasive grains satisfies 20 μm≦D≦60 μm.
 11. The rare-earth alloycutting method of claim 9, wherein the core wire has a diameter of about0.12 mm to about 0.2 mm.
 12. The rare-earth alloy cutting method ofclaim 9, wherein the resin layer is made of one of a phenol resin, anepoxy resin and a polyimide resin.
 13. The rare-earth alloy cuttingmethod of claim 9, wherein the step of moving the wire saw includes thestep of moving the wire saw on a plurality of rollers, and each of theplurality of rollers includes a polymer layer on which a guide groove isprovided, the guide groove has a pair of sloped surfaces, at least oneof the sloped surfaces of the guide groove defines an angle of about 25degrees to less than about 45 degrees with respect to a radial directionof the roller, and the wire is passed between the sloped surfaces of theguide groove.
 14. The rare-earth alloy cutting method of claim 9,wherein the rare-earth alloy is an R—Fe—B based rare-earth sinteredalloy.
 15. The rare-earth alloy cutting method of claim 14, wherein therare-earth alloy is an Nd—Fe—B based rare-earth sintered alloy.
 16. Therare-earth alloy cutting method of claim 9, wherein the wire saw is fedwith a tension about 25 N to about 35 N while being moved to cut therare-earth alloy.
 17. The rare-earth alloy cutting method of claim 9,wherein the coolant is at least approximately 70 wt % water.
 18. Therare-earth alloy cutting method of claim 9, wherein a temperature of thecoolant is about 1 5° C. to about 35° C.
 19. The rare-earth alloycutting method of claim 9, wherein a temperature of the coolant is about1 5° C. to about 35° C.
 20. The rare-earth alloy cutting method of claim9, wherein the coolant has at least one of a surfactant, a syntheticlubricant, an antifoaming agent, a pH of about 8 to about 11, and a rustpreventive material.
 21. The rare-earth alloy cutting method of claim 9,wherein the wire saw is made of one of a piano wire, Ni—Cr alloy, Fe—Nialloy, W, Mo, and a bundle of nylon fibers.
 22. The rare-earth alloycutting method of claim 9, wherein the abrasive grains are made of oneof diamond, SiC, B, C and CBN.
 23. A method of cutting a rare-earthalloy with a wire saw obtained by fixing abrasive grains on a core wirewith a resin layer, the method comprising the step of moving the wiresaw while a portion of the rare-earth alloy being cut by the wire saw isimmersed in a coolant, which is mainly composed of water and has akinetic friction coefficient of about 0.1 to about 0.3 at approximately25° C. with respect to the rare-earth alloy, thereby cutting therare-earth alloy; wherein in the wire saw, an average distance betweentwo of the abrasive grains, which are adjacent to each other in a lengthdirection, is about 150% to less than about 400% of the average grainsize of the abrasive grains, an average height of portions of theabrasive grains, protruding from the surface of the resin layer, isabout 70% or less of the average grain size of the abrasive grains, anda thickness deviation percentage of the resin layer with respect to thecore wire is about 40%.
 24. The rare-earth alloy cutting method of claim23, wherein the average grain size D of the abrasive grains satisfies 20μm≦D≦60 μm.
 25. The rare-earth alloy cutting method of claim 23, whereinthe core wire has a diameter of about 0.12 mm to about 0.2 mm.
 26. Therare-earth alloy cutting method of claim 23, wherein the resin layer ismade of one of a phenol resin, an epoxy resin and a polyimide resin. 27.The rare-earth alloy cutting method of claim 23, wherein the step ofmoving the wire saw includes the step of moving the wire saw on aplurality of rollers, and each of the plurality of rollers includes apolymer layer on which a guide groove is provided, the guide groove hasa pair of sloped surfaces, at least one of the sloped surfaces of theguide groove defines an angle of about 25 degrees to less than about 45degrees with respect to a radial direction of the roller, and the wireis passed between the sloped surfaces of the guide groove.
 28. Therare-earth alloy cutting method of claim 23, wherein the rare-earthalloy is an R—Fe—B based rare-earth sintered alloy.
 29. The rare-earthalloy cutting method of claim 28, wherein the rare-earth alloy is anNd—Fe—B based rare-earth sintered alloy.
 30. The rare-earth alloycutting method of claim 23, wherein the wire saw is fed with a tensionabout 25 N to about 35 N while being moved to cut the rare-earth alloy.31. The rare-earth alloy cutting method of claim 23, wherein the coolantis at least approximately 70 wt % water.
 32. The rare-earth alloycutting method of claim 23, wherein a temperature of the coolant isabout 15° C. to about 35° C.
 33. The rare-earth alloy cutting method ofclaim 23, wherein a temperature of the coolant is about 15° C. to about35° C.
 34. The rare-earth alloy cutting method of claim 23, wherein thecoolant has at least one of a surfactant, a synthetic lubricant, anantifoaming agent, a pH of about 8 to about 11, and a rust preventivematerial.
 35. The rare-earth alloy cutting method of claim 23, whereinthe wire saw is made of one of a piano wire, Ni—Cr alloy, Fe—Ni alloy,W, Mo, and a bundle of nylon fibers.
 36. The rare-earth alloy cuttingmethod of claim 23, wherein the abrasive grains are made of one ofdiamond, SiC, B, C and CBN.